One of the key components of any computer system is a place to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disk drive. The most basic parts of a disk drive are a disk that is rotated, an actuator that moves a transducer to various locations over the disk, and electrical circuitry that is used to write and read data to and from the disk. The disk drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disk surface. A microprocessor controls most of the operations of the disk drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disk.
The transducer is typically housed within a small ceramic block. The small ceramic block is passed over the disk in a transducing relationship with the disk. The transducer can be used to read information representing data from the disk or write information representing data to the disk. When the disk is operating, the disk is usually spinning at relatively high RPM. These days common rotational speeds are 5100 and 7200 RPM. Rotational speeds of 10,000 RPM and higher are contemplated for the future. These high rotational speeds place the small ceramic block in high air speeds. The small ceramic block, also referred to as a slider, is usually aerodynamically designed so that it flies over the disk. The best performance of the disk drive results when the ceramic block is flown as closely to the surface of the disk as possible. Today's small ceramic block or slider is designed to fly on a very thin layer of gas or air. In operation, the distance between the small ceramic block and the disk is very small. Currently "fly" heights are about 12 microinches. In some disk drives, the ceramic block does not fly on a cushion of air but rather passes through a layer of lubricant on the disk.
Information representative of data is stored on the surface of the memory disk. Disk drive systems read and write information stored on tracks on memory disks. Transducers, in the form of read/write heads, located on both sides of the memory disk, read and write information on the memory disks when the transducers are accurately positioned over one of the designated tracks on the surface of the memory disk. The transducer is also said to be moved to a target track. As the memory disk spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the memory disk. Similarly, reading data on a memory disk is accomplished by positioning the read/write head above a target track and reading the stored material on the memory disk. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disk drives, the tracks are a multiplicity of concentric circular tracks. In other disk drives, a continuous spiral is one track on one side of a disk drive. Servo feedback information is used to accurately locate the transducer. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
One of the most critical times during the operation of a disk drive is just before the disk drive shuts down. The small ceramic block is typically flying over the disk at a very low height when shutdown occurs. In the past, the small block was moved to a non data area of the disk where it literally landed and skidded to a stop. Problems arise in such a system. When disks were formed with a smooth surface, stiction would result between the small ceramic block and the head. In some instances the force due to stiction was so strong that it virtually ripped the head off the suspension. Amongst the other problems was a limited life of the disk drive. Each time the drive was turned off another start stop contact cycle would result. After many start stop contacts, the small ceramic block may chip or produce particles. The particles could eventually cause the disk drive to fail. When shutting down a disk drive, several steps are taken to help insure that the data on the disk is preserved. In general, the actuator assembly is moved so that the transducers do not land on the portion of the disk that contains data. There are many ways to accomplish this. A ramp on the edge of the disk is one design method that has gained industry favor more recently. Disk drives with ramps are well known in the art. U.S. Pat. No. 4,933,785 issued to Morehouse et al. is one such design. Other disk drive designs having ramps therein are shown in U.S. Pat. Nos. 5,455,723, 5,235,482 and 5,034,837.
Typically, the ramp is positioned to the side of the disk. A portion of the ramp is positioned over the disk itself. In operation, before power is actually shut off, the actuator assembly swings the suspension or another portion of the actuator assembly up the ramp to a park position at the top of the ramp. When the actuator assembly is moved to a position where parts of the suspension are positioned on the top of the ramp, the sliders or ceramic blocks do not contact the disk. Commonly, this procedure is known as unloading the heads. Unloading the heads helps to insure that data on the disk is preserved since, at times, unwanted contact between the slider and the disk results in data loss on the disk.
When the disk is not being used this may be a critical time. If the disk drive is shock loaded, it is most desirable to have the actuator arm stay in its parked position with the suspensions atop the ramp. Sometimes, when a disk drive is shock loaded, the actuator arm leaves the ramp and the small ceramic block or slider contacts or slaps the disk. This may cause an immediate loss of data or may result in generation of particles. Particle generation may result in a later loss of data.
Startup of a disk drive with a ramp is another critical time. Startup includes moving the actuator assembly so that the suspension slides down the ramp and so that the slider flies when it gets to the bottom of the ramp. It is desirable to have the ramp well lubricated so that the suspension assembly goes down the ramp easily. It is also desirable to control the speed at which the suspension goes down the ramp so as to prevent contact between the slider and the disk. If the velocity can be controlled as the suspension moves down the ramp, the slider will not contact the disk.
U.S. Pat. No. 4,864,437 issued to Couse et al. teaches one way of controlling the velocity of the slider as it moves down a ramp. In Couse et al., the voltage across a voice coil motor is monitored and controlled. The voltage across the voice coil motor includes a small component of the total voltage known as Back EMF. A voice coil motor includes magnets and an actuator coil. When the actuator coil cuts a magnetic field, Back EMF is generated. The Back EMF varies as a function of the velocity of the actuator coil through the magnetic field produced by the magnets of the voice coil motor and, presumably, as a function of the velocity of the actuator down the ramp. Thus, it is possible to get an estimate of the rotational velocity of the actuator from the Back EMF of the actuator motor. The design of the velocity control in Couse et al. also has problems. Most problematic is that the Back EMF is a very small component of the total voltage across the coil of the actuator. This component will also become smaller as additional current is passed through the coil. The Back EMF signal is also prone to noise. In short, since the Back EMF component of the voltage across the actuator is small and prone to noise, it does not always reliably reflect the actual velocity of the slider. In addition, as the operating temperature of the disk drive increases, the noise level increases making the Back EMF an even smaller component and even more prone to noise. If there happens to be an error indicating that the velocity is slower than it actually is, then an increase in the actuator coil current may cause the velocity of the slider down the ramp to increase to the point where the slider will contact the surface of the disk.
There is a need for a method and apparatus that can maintain the actuator arm assembly in a parked position even when power is removed from the disk drive. There is also a need to allow the suspension to slide up and down the ramp easily. There is a further need for a method and apparatus that may be used to effectively control the velocity of the actuator as it goes up and down the ramp.